1. Field of the Invention
The present invention relates to methods and apparatus for adaptively performing algebraic interference cancellation and, more particularly, to techniques for adaptively determining the angular location of interference signals and selectively rejecting the interference signals without distorting a received signal of interest.
2. Description of the Related Art
Phased array antennas consist of an array of individual antenna elements arranged in a particular manner to cooperatively transmit and receive directed beams of electromagnetic energy. An array antenna beam pattern, which typically includes a main lobe and side lobes, defines the angular dependence of the array gain. The shape and direction of an array antenna beam pattern are determined by the relative phases and amplitudes applied at the individual antenna elements that constitute the array via a process referred to as beamforming. By adjusting the relative phases of the antenna elements, the main lobe of the antenna beam pattern can be steered over a range of different directions to transmit a signal in a selected direction or to receive a signal arriving from a particular direction. When receiving a signal, received power is maximized by pointing the main lobe of the array antenna beam pattern in the direction of a source of a signal of interest.
Ideally, the signal of interest is aligned with the boresight of the antenna beam pattern to maximize the received signal strength. Nevertheless, interference signals in the sidelobes or main lobe of the antenna beam pattern may also be received along with the signal of interest. Adaptive interference cancellation schemes for phased array antennas have been in use since the 1960s (see, for example, S. Applebaum, “Adaptive Arrays,” Syracuse University Research Corp. Report, SPL TR 66-1, August 1966, and B. Widrow at al., “Adaptive Antenna Systems,” Proc. IEEE, Vol. 55, December 1967). Many variations of such schemes exist, but all of them involve the following process. At each sampling instant, a beamforming operation is performed on the set of array outputs. The array outputs present a vector of complex numbers representing a signal of interest and possibly interference from one or more interfering sources, e.g., jammers. The weights used in the beamforming operation, which are calculated in an adaptive process, yield a beam pattern that is deliberately distorted in a manner such that nulls are imposed on the pattern at the angles corresponding to the directions of the interfering sources. As discussed in Hudson, “Principles of Adaptive Arrays,” pp. 39-48, Peter Peregrinus, London, 1981, operating with the weights yielded by an optimized adaptive process is equivalent to a beamforming operation in which the beamsteering vectors have been projected into a space orthogonal to the interference directions. The orthogonality is manifested by the nulls in the beam pattern.
The adaptive beamforming/nulling (ABF) operation can be thought of as a process in which a beampattern representing signal plus interference is formed, from which a beampattern representing interference alone is subtracted, as suggested by Hudson and by Monzingo in “Introduction to Adaptive Arrays,” Wiley-Interscience, New York, 1980. As such, when the angular separation between signal and interference becomes less than the width of the main lobe, the interference and signal beams begin to overlap, and rejection of interference cannot take place without also attenuating and distorting the signal of interest, which becomes more severe and pronounced as the signal/interference angular separation decreases. Thus, interference in the main lobe of the antenna beam presents a severe problem for ABF systems, and the interference rejection resolution capability is limited by the resolution of the antenna array. Also, since ABF schemes achieve interference rejection by distortion of the array beampatterns, distortions in the output time function representing the signal is unavoidable. Furthermore, since the beamforming process employed is irreversible, it is not possible to correct for the signal distortion. Consequently, it would be desirable to provide improved interference rejection, particularly for interference sources located in the main beam, without distorting the signal of interest.